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Hardware solutions for ITER divertor Thomson scattering
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Hardware solutions for ITER divertor Thomson scattering Eugene Mukhin a,b,∗ , Philip Andrew c , Nikita Babinov a , Michele Bassan c , Alexander Bazhenov a , Ivan Bukreev a , Alexander Chernakov d , Anton Chernakov d , Artem Dmitriev a , Victor Yukish e , Michael Kochergin a,c , Alexander Koval a,b , Gleb Kurskiev a,b , Andrey Litvinov a,b , Vladimir Nelyubov e , Alexey Razdobarin a , Dmitry Samsonov a , Vladimir Semenov a , Vladimir Solokha a , Valery Solovey a , Sergey Tolstyakov a,b , Michael Walsh c a
Ioffe Institute, 194021, St.Petersburg, Russia Technoexan, AO, 194021, St.Petersburg, Russia ITER Organization, CS 90 046, 13067, St. Paul Lez Durance Cedex, France d Spectral-Tech, ZAO, 194223, St. Petersburg, Russia e KOMNET, OOO, 394036, Voronezh, Russia b c
a r t i c l e
i n f o
Article history: Received 30 September 2016 Received in revised form 19 February 2017 Accepted 9 June 2017 Available online xxx Keywords: ITER Diagnostic hardware Thomson scattering Divertor Laser Polychromator Piezo actuator
a b s t r a c t The ITER divertor Thomson scattering (TS) system has been significantly upgraded in the last few years in terms of equipment and experimental setup design. In this paper, we are focusing on lasers, digital filter polychromator and piezo actuators, which could be also useful for other diagnostics. Capabilities of the probing and calibration lasers are specified and compared against ITER requirements. Key features of the digital polychromator design for TS applications are presented and obtained signal-to-noise ratios are discussed. The main reason to use piezo actuators for in-vessel remotely controlled devices is high radiation and magnetic resistance of piezo elements. Piezo actuators of inertial and ultrasonic types were produced and their promising performance for fusion devices was demonstrated. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The ITER divertor Thomson scattering (TS) diagnostic is designed to measure the local Te and ne in the outer leg of the divertor providing the link between upstream and target electron parameter measurements (Table 1). It is a single-pass multipoint TS system. In-vessel optical elements are challenging for all optical diagnostics in ITER. For the divertor TS, the most challenging optical elements are the mirrors launching laser beams into the plasma. Therefore, the optical layout based on several interchangeable probing chords with the same collection optics should improve reliability. All the mirrors are protected from deposits/sputtering by thin quartz windows. Laser beams launched by First Laser Mirror (FLM) 1 (yellow laser paths, see Fig. 1) are very close to the separatrix and, hence,
∗ Corresponding author at: Ioffe Institute, 194021, St.Petersburg, Russia. E-mail address: [email protected] (E. Mukhin).
coincide with the main stream to the outer divertor leg. This mirror is the most informative but also the most deposit-prone. Laser beams launched by FLM 2 (green laser paths) and FLM 3 (blue laser paths) are less informative but also less deposit-prone as situated further from the target. The probing beams launched from FLM 3 are eligible only for measurements in a strike point vicinity. The relative location of probing and viewing chords with magnetic field lines structure is shown in Fig. 1. TS for divertor plasma diagnostics was used in DIIID [2], MAST [3] and NSTX [4]. While the basic TS principles will be similar to those in modern plasma devices, advanced technical capabilities are required to study ITER divertor plasmas. The in-vessel frontend optical components will be quite near to the divertor plasma and several meters from the vacuum boundary, hence vibrations or mechanical deformations and/or displacements during cooling/heating are possible. Since the diagnostic equipment will be close to the source of impurities due to physical/chemical sputtering of the divertor targets, the diagnostic optics should be protected
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Please cite this article in press as: E. Mukhin, et al., Hardware solutions for ITER divertor Thomson scattering, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.06.014
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2 Table 1 DTS measurement capabilities [1].
Divertor electron parameters
Parameter
Range
Frequency
Accuracy
ne m−3 Te eV
1019 –1022 1–200 0.3–1
50 Hz 50 Hz
20% 20% 0.2 eV
Table 2 Lasers developed for ITER divertor TS. Parameter
Nd:YAG
Nd:YLF
Nd:YAG
Task Wavelength, nm Energy, J Rep. rate, Hz
probing 1064 2 50
calibration 5–500 eV 1047 2 5
calibration 500–5 keV 946 ∼1 50
from deposits. Special hardware solutions, being practical and reliable, should also take into account highly hostile environment, possible degradation of the collection optics transmission spectra and restricted access through the narrow gap between divertor cassettes. In this paper, we attempted to address all these problems: divertor TS lasers and their characteristics (Section 2); capabilities of digital polychromator and its innovative design rationale (Section 3); piezo actuators specially developed for ITER in-vessel (Section 4).
Fig. 1. Probing chords and collection cones of divertor TS.
2. Laser system Laser capabilities are critical for all TS diagnostics. Currently, the divertor TS concept is based on three different laser types: 1064 nm Nd:YAG probing laser and two lasers (1047 nm Nd:YLF and 946 nm Nd:YAG) for calibration [1] (see Table 2). Although the wavelength 1064 nm is commonly used, the TS system developed for ITER requires a special approach. In particular, the laser should be highly reliable to generate at least 108 pulses with relatively high repetition rate (tens Hz) at steady state. Q-switched Nd:YAG 1064 nm laser (2 J, 50 Hz) with close to diffraction-limited divergence [5] matched with optical delivery scheme transmitting the nearfield image to the laser launcher minimizes energy loss and uniformly distribute intensity over the frontend laser optics. A diode-pumped version of the laser with diode-seeded master oscillator is under development; laser diode pumping instead of flash-lamp is expected to improve the near-field energy distribution across the laser footprint as well as considerably increase time between maintenance. The seeding master oscillator helps to stabilize the shape of laser pulses, thus simplifying interpretation of experimental results. At calibration, the TS spectra of different laser wavelengths scattered in the plasma should be compared to assess degradation of the collection optics spectral transmission. The 1064 nm laser when combined with 1047 nm or 946 nm lasers can be used for calibration within Te ∼ 5 eV–500 eV or 500 eV–5 keV, respectively [1]. The spectral range is expanded up to several keV to retain the capacity for measurement of the divertor plasma during ELM activity in which toroidally asymmetric structures carrying plasma with temperature characteristic of the H-mode pedestal can be present. While 1047 nm Nd:YLF laser technology is well established, 946 nm Nd:YAG laser with similar parameters is a challenge, since for lasing on quasi-three-level transitions [6], a population inversion can be achieved at high pump intensity only. Moreover, conventional optical schemes are not applicable because the stimulated emission cross-section for 946 nm in the Nd:YAG element is ∼5 times smaller than for 1064 nm, and parasitic oscillations at the higher gain transition must be suppressed. Our estimations and preliminary experiments demonstrate feasibility of the diode-pumped 946 nm lasers with pulse energy as high as 1–2 J and repetition
Fig. 2. Nd:YAG 1064 nm laser pulse and the corresponding recorded TS signal. The laser pulse was detected using high-speed Si diode STANDA 11HSP-V2 (red curve) and the TS signal (blue curve) – using Digital polychromator (see below).
rate of 50–100 Hz. Additionally, our 1064 nm laser has pulse duration ∼3 ns Full-Width at Half-Maximum (FWHM) with the rise time of ∼1 ns (Fig. 2). Such short lasing times matched with high-speed electronics (see the blue curve in Fig. 2) help to reduce contribution from the quasi-continuous plasma background and black body radiation from the ITER divertor targets. Furthermore, the higher temporal resolution combined with short rise time facilitates separation of TS signal and stray light. 3. Digital polychromator TS polychromators based on band-pass-filters and avalanche photodiodes (APDs) have a high throughput and are routinely used for TS diagnostics in fusion devices [7]. The recently presented [8] digital filter polychromator equipped with high-speed APD-based detection system can be easily stacked in a standard 19” rack (up to 15 per rack). Owing to compactness and ultra-low energy consumption of modern computers and analog-to-digital converters (ADC), both optical unit and data processing electronics with Gigabit optical Ethernet were combined in one stand-alone device (see Fig. 3). Such all-in-one design with fiber input and digital optical output is expected to be much more resistant to electromagnetic disturbances. Besides, the equipment can be integrated into the control/data acquisition system more easily. APDs are equipped with pHEMT-based (pHEMT – pseudomorphic High Electron Mobil-
Please cite this article in press as: E. Mukhin, et al., Hardware solutions for ITER divertor Thomson scattering, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.06.014
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4. Piezo actuators for ITER in-vessel
Fig. 3. Digital filter polychromator specially developed for TS.
Fig. 4. RRS signals measured in the 1 st spectral channel for different Nitrogen pressures.
ity Transistor) transimpedance preamplifiers specially designed for wideband, ultra-low noise amplification [9]. These preamplifiers are able to compensate slowly variable signals (background radiation); therefore, a full ADC dynamic range can be used to digitize TS signals. The integration time 3–4 ns was optimized for ∼3 ns FWHM TS signals. The acquisition system based on DRS4 chip [10] provides 12-bit digitizing with ∼0.25 mV amplitude resolution and ∼5 GHz sampling rate. Such high sampling rates combined with oscilloscope mode enable flexible processing of digitized signals as opposed to charge-sensitive analog methods. For sensitivity calibration we used rotational Raman scattering (RRS) generated by 1064 nm laser in nitrogen gas (4–1330 Pa). Fig. 4 shows scattering signal intensity versus N2 pressure. Signals for each pressure value were measured 100 times as shown by grey points. Mean values (red crosses in Fig. 4) demonstrate good linear correlation throughout the pressure range. The dispersion of Raman scattering signals (see Fig. 4) gives information about overall detection accuracy for different signal values, which includes superposition of statistical signal noise, amplifier noise and other disturbing influences on the signal. The measured level of intrinsic noise corresponds to ∼16 photoelectrons at the detector input (before avalanche gain) and demonstrates a dynamic range of 103 under the maximum digitized signal of 16,000 photoelectrons. According to this experiment, for signals of 100 photoelectrons and above, the noise of the RRS signal is determined mainly by statistical noise and constant excess noise factor. The excess noise factor F, defined as a ratio of the signal-to-noise ratios at the input of the multiplication process to that of the measured signal, was determined as F ∼ 4.7 with accuracy F ∼ 0.15 of the entire detector unit based on 1.5 mm APD Hamamatsu S11519 under the avalanche gain M = 100 for = 1064 nm.
Future fusion devices will require machinery to operate under vacuum conditions, radiation fluxes (up to ∼5 × 1019 neutron/cm2 fluence over ITER’s lifetime), heating up to 250 ◦ C and strong magnetic field (several Tesla). Conventional electric motors are not eligible for the in-vessel ITER applications (e.g. front divertor diagnostic rack, see Fig. 1), particularly under strong magnetic fields. Therefore, new actuator designs are required. The actuators based on the piezo effect seem ideal for fusion applications. Interestingly, that regular benefits of the piezo devices (submicron resolution, small size and noiseless operation) are less critical, while their high radiation and temperature resistance, as well as absolute insensitivity to magnetic fields become important. Mechanical movements and torques in the piezo actuators occur due to the frictional contact force between stator and rotor or slider. Since dry friction between similar materials in vacuum can easily lead to cold welding in the friction pair, specially matched dissimilar materials were coupled in the ITER intendent actuators. Different temperature coefficients of ceramic and metal linear expansions are compensated by gaps or by thermally-aged springs made of Inconel 718. Two types of piezoelectric actuators were developed for ITER applications – inertial stick-slip (SS) and ultrasonic (US, or resonant standing wave motor). SS type is based on a cyclic change of static and sliding friction resulting from applying an asymmetric sawtooth voltage to the piezoelectric element, which is friction coupled with a massive rotor. Because of inertia, the rotor must be put into motion during the saw-tooth slow slope and remains at its position on the fast slope. In the US motor, the rotor is driven in rotation with a piezoelectric vibrator that oscillates at a resonance frequency of 100–200 kHz and leads to an elliptical motion of a stator driving foot converted into the linear motion of a mover (or rotation of a rotor) pressed on the driving foot. The US piezo engine specificity is operation with fast transient responses and moderate loads as opposed to SS engines, which in some versions can move hundreds of kilograms but per minutes (or even per hours). In the developed piezo actuators, the following technical solutions have been implemented to satisfy all the ITER requirements: – High temperature piezo electric ceramic with the Curie temperature ≥300 ◦ C; – Alumina (Al2 O3 ), silicon nitride (Si3 N4 ) and ceramic mineral glue as insulating components; – Si3 N4 bearings with separators of AISI 304; – No lubricants and no soldering; – Spring elements of Inconel 718; – Friction pair in inertial version is Si3 N4 (rotor) and Inconel 718 (stator); – Friction pair in supersonic version is Si3 N4 (rotor) and piezo element (stator). Several designs of ultrasonic and inertial actuators were developed and successfully tested in vacuum at temperatures of 20 ◦ C and 200 ◦ C after annealing during 24 h at 350 ◦ C. Figs. 5a, b show the last versions of ultrasonic and inertial actuators. Fig. 6 demonstrates the measured relationship between torque and angular velocity at the start of acceleration of the US actuator testbed with the attached dummy shutter (the load torque of 0.01 N × m). The load has been chosen equal to that of the divertor TS FLM shutter. The maximal measured angular velocity is ∼8 rotations per second at 20 ◦ C vs ∼6.5 at 200 ◦ C. The maximal torque of ∼0.2 N × m and angular velocity 2 rotations per minute were measured for the SS actuator testbed featured with 6 piezo elements and the length of friction surface equal to circumference of Ø 40 mm rotor. The torque is expected
Please cite this article in press as: E. Mukhin, et al., Hardware solutions for ITER divertor Thomson scattering, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.06.014
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Fig. 5. Piezo actuators developed for ITER application based on Si3 N4 bearings of Ø 40 mm for SS actuator and Ø17 for US actuator. (a) US dimensions Ø54 × 15 mm; weight 160 g. (b) SS dimensions Ø84 × 24 mm; weight 750 g.
lated plasma background and to separate TS signals from signals of stray light scattered on nearby constructions. It is expected that the diode-pumped laser under development will improve the nearfield energy distribution in the laser footprint and further increase time between maintenance. TS polychromators based on bandpass-filters, high-speed APD-based detection system and ∼5-GHz, 12-bit ultra-low power ADC has an integrated design combining the optical unit, digitizer and compact computer unit for data processing with Gigabit optical Ethernet in one stand-alone device. Such a device with fiber input and digital optical output is expected to be quite resistant to electromagnetic disturbances. The experiments with Raman scattering show that for signals of 100 photoelectrons and above, the noise of the RRS signal is determined mainly by statistical noise and constant excess noise factor. The ultrasonic and inertial piezo motor designs satisfy all the ITER requirements. The piezo motor prototypes were tested for key specifications, although certain efforts are necessary for their specific adaptation and validation according to particular requirements. Acknowledgments This report supported in part by Rosatom (contract – N.4k.52.9Б.14.1002) was prepared as an account of work for the ITER Organization. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.
Fig. 6. Parameters of the developed US and SS piezo actuators: a – Acceleration of the US piezo actuator with the attached dummy shutter (load torque of 0.02 N × m); b – Relationship between torque and angular velocity for the developed SS actuator.
to be proportional and angular velocity – inversely proportional to rotor diameters of SS piezo actuators. It is obvious that, for both SS and US piezo actuators, rotors with larger diameter (circumference) are able to allocate more piezo elements, with respective increase of torques. Thus, to improve performance of piezo actuators (both SS and US), diameters and/or number of rotors should be adjusted. 5. Conclusions This paper focuses on innovative hardware – 1064 nm Nd:YAG; 946 nm Nd:YAG and 1047 nm Nd:YLF lasers; digital polychromator; US and SS piezo actuators. All these prototypes being developed for ITER divertor TS can readily be used for other ITER applications as well as in other tokamaks. The developed 1064 nm Nd:YAG laser operates at 100 Hz in steady state with pulse duration as low as ∼3 ns (FWHM). Such short lasing times help to reduce the accumu-
References [1] E.E. Mukhin, et al., Physical aspects of divertor Thomson scattering implementation on ITER, Nucl. Fusion 54 (043007) (2014) 13. [2] T.N. Carlstrom, et al., 1997 Initial operation of the divertor Thomson scattering diagnostic on DIII-D Rev, Sci. Instrum. 68 (2016) 1195. [3] J. Hawke, et al., Outline of optical design and viewing geometry for divertor Thomson scattering on MAST upgrade, JINST 8 (2013) C11010. [4] A.G. McLean, Conceptual design of a divertor Thomson scattering diagnostic for NSTX-U, Rev. Sci. Instrum. 85 (11) (2014) 11E825. [5] E.E. Mukhin, et al., The ITER divertor Thomson scattering system: engineering and advanced hardware solutions, JINST 7 (2012) C02063. [6] T.Y. Fan, R.L. Byer, Modeling and CW operation of a quasi-three-level 946 nm Nd:YAG laserJ, J. Quantum Electron. 23 (1987) 605–612. [7] T.N. Carlstrom, et al., A compact, low cost, seven channel polychromator for Thomson scattering measurements, Rev. Sci. Instrum. 61 (1990) 2858. [8] V. Solokha, et al, Digital filter polychromator for Thomson scattering applications, in: 7th International Workshop & Summer School on Plasma Physics, Kiten, 2016 (in press of Journal of Physics: Conference Series). [9] M. Nosal Zbigniew, Performance analysis of transimpedance amplifiers in various technologies, in: 12th GAAS Symposium, Amsterdam, 2004. [10] S. Ritt, et al., Application of the DRS chip for fast waveform digitizing, NIMA 623 (2010) 486–488.
Please cite this article in press as: E. Mukhin, et al., Hardware solutions for ITER divertor Thomson scattering, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.06.014
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FUSION-9646; No. of Pages 4
Fusion Engineering and Design xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Hardware solutions for ITER divertor Thomson scattering Eugene Mukhin a,b,∗ , Philip Andrew c , Nikita Babinov a , Michele Bassan c , Alexander Bazhenov a , Ivan Bukreev a , Alexander Chernakov d , Anton Chernakov d , Artem Dmitriev a , Victor Yukish e , Michael Kochergin a,c , Alexander Koval a,b , Gleb Kurskiev a,b , Andrey Litvinov a,b , Vladimir Nelyubov e , Alexey Razdobarin a , Dmitry Samsonov a , Vladimir Semenov a , Vladimir Solokha a , Valery Solovey a , Sergey Tolstyakov a,b , Michael Walsh c a
Ioffe Institute, 194021, St.Petersburg, Russia Technoexan, AO, 194021, St.Petersburg, Russia ITER Organization, CS 90 046, 13067, St. Paul Lez Durance Cedex, France d Spectral-Tech, ZAO, 194223, St. Petersburg, Russia e KOMNET, OOO, 394036, Voronezh, Russia b c
a r t i c l e
i n f o
Article history: Received 30 September 2016 Received in revised form 19 February 2017 Accepted 9 June 2017 Available online xxx Keywords: ITER Diagnostic hardware Thomson scattering Divertor Laser Polychromator Piezo actuator
a b s t r a c t The ITER divertor Thomson scattering (TS) system has been significantly upgraded in the last few years in terms of equipment and experimental setup design. In this paper, we are focusing on lasers, digital filter polychromator and piezo actuators, which could be also useful for other diagnostics. Capabilities of the probing and calibration lasers are specified and compared against ITER requirements. Key features of the digital polychromator design for TS applications are presented and obtained signal-to-noise ratios are discussed. The main reason to use piezo actuators for in-vessel remotely controlled devices is high radiation and magnetic resistance of piezo elements. Piezo actuators of inertial and ultrasonic types were produced and their promising performance for fusion devices was demonstrated. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The ITER divertor Thomson scattering (TS) diagnostic is designed to measure the local Te and ne in the outer leg of the divertor providing the link between upstream and target electron parameter measurements (Table 1). It is a single-pass multipoint TS system. In-vessel optical elements are challenging for all optical diagnostics in ITER. For the divertor TS, the most challenging optical elements are the mirrors launching laser beams into the plasma. Therefore, the optical layout based on several interchangeable probing chords with the same collection optics should improve reliability. All the mirrors are protected from deposits/sputtering by thin quartz windows. Laser beams launched by First Laser Mirror (FLM) 1 (yellow laser paths, see Fig. 1) are very close to the separatrix and, hence,
∗ Corresponding author at: Ioffe Institute, 194021, St.Petersburg, Russia. E-mail address: [email protected] (E. Mukhin).
coincide with the main stream to the outer divertor leg. This mirror is the most informative but also the most deposit-prone. Laser beams launched by FLM 2 (green laser paths) and FLM 3 (blue laser paths) are less informative but also less deposit-prone as situated further from the target. The probing beams launched from FLM 3 are eligible only for measurements in a strike point vicinity. The relative location of probing and viewing chords with magnetic field lines structure is shown in Fig. 1. TS for divertor plasma diagnostics was used in DIIID [2], MAST [3] and NSTX [4]. While the basic TS principles will be similar to those in modern plasma devices, advanced technical capabilities are required to study ITER divertor plasmas. The in-vessel frontend optical components will be quite near to the divertor plasma and several meters from the vacuum boundary, hence vibrations or mechanical deformations and/or displacements during cooling/heating are possible. Since the diagnostic equipment will be close to the source of impurities due to physical/chemical sputtering of the divertor targets, the diagnostic optics should be protected
http://dx.doi.org/10.1016/j.fusengdes.2017.06.014 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: E. Mukhin, et al., Hardware solutions for ITER divertor Thomson scattering, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.06.014
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FUSION-9646; No. of Pages 4
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2 Table 1 DTS measurement capabilities [1].
Divertor electron parameters
Parameter
Range
Frequency
Accuracy
ne m−3 Te eV
1019 –1022 1–200 0.3–1
50 Hz 50 Hz
20% 20% 0.2 eV
Table 2 Lasers developed for ITER divertor TS. Parameter
Nd:YAG
Nd:YLF
Nd:YAG
Task Wavelength, nm Energy, J Rep. rate, Hz
probing 1064 2 50
calibration 5–500 eV 1047 2 5
calibration 500–5 keV 946 ∼1 50
from deposits. Special hardware solutions, being practical and reliable, should also take into account highly hostile environment, possible degradation of the collection optics transmission spectra and restricted access through the narrow gap between divertor cassettes. In this paper, we attempted to address all these problems: divertor TS lasers and their characteristics (Section 2); capabilities of digital polychromator and its innovative design rationale (Section 3); piezo actuators specially developed for ITER in-vessel (Section 4).
Fig. 1. Probing chords and collection cones of divertor TS.
2. Laser system Laser capabilities are critical for all TS diagnostics. Currently, the divertor TS concept is based on three different laser types: 1064 nm Nd:YAG probing laser and two lasers (1047 nm Nd:YLF and 946 nm Nd:YAG) for calibration [1] (see Table 2). Although the wavelength 1064 nm is commonly used, the TS system developed for ITER requires a special approach. In particular, the laser should be highly reliable to generate at least 108 pulses with relatively high repetition rate (tens Hz) at steady state. Q-switched Nd:YAG 1064 nm laser (2 J, 50 Hz) with close to diffraction-limited divergence [5] matched with optical delivery scheme transmitting the nearfield image to the laser launcher minimizes energy loss and uniformly distribute intensity over the frontend laser optics. A diode-pumped version of the laser with diode-seeded master oscillator is under development; laser diode pumping instead of flash-lamp is expected to improve the near-field energy distribution across the laser footprint as well as considerably increase time between maintenance. The seeding master oscillator helps to stabilize the shape of laser pulses, thus simplifying interpretation of experimental results. At calibration, the TS spectra of different laser wavelengths scattered in the plasma should be compared to assess degradation of the collection optics spectral transmission. The 1064 nm laser when combined with 1047 nm or 946 nm lasers can be used for calibration within Te ∼ 5 eV–500 eV or 500 eV–5 keV, respectively [1]. The spectral range is expanded up to several keV to retain the capacity for measurement of the divertor plasma during ELM activity in which toroidally asymmetric structures carrying plasma with temperature characteristic of the H-mode pedestal can be present. While 1047 nm Nd:YLF laser technology is well established, 946 nm Nd:YAG laser with similar parameters is a challenge, since for lasing on quasi-three-level transitions [6], a population inversion can be achieved at high pump intensity only. Moreover, conventional optical schemes are not applicable because the stimulated emission cross-section for 946 nm in the Nd:YAG element is ∼5 times smaller than for 1064 nm, and parasitic oscillations at the higher gain transition must be suppressed. Our estimations and preliminary experiments demonstrate feasibility of the diode-pumped 946 nm lasers with pulse energy as high as 1–2 J and repetition
Fig. 2. Nd:YAG 1064 nm laser pulse and the corresponding recorded TS signal. The laser pulse was detected using high-speed Si diode STANDA 11HSP-V2 (red curve) and the TS signal (blue curve) – using Digital polychromator (see below).
rate of 50–100 Hz. Additionally, our 1064 nm laser has pulse duration ∼3 ns Full-Width at Half-Maximum (FWHM) with the rise time of ∼1 ns (Fig. 2). Such short lasing times matched with high-speed electronics (see the blue curve in Fig. 2) help to reduce contribution from the quasi-continuous plasma background and black body radiation from the ITER divertor targets. Furthermore, the higher temporal resolution combined with short rise time facilitates separation of TS signal and stray light. 3. Digital polychromator TS polychromators based on band-pass-filters and avalanche photodiodes (APDs) have a high throughput and are routinely used for TS diagnostics in fusion devices [7]. The recently presented [8] digital filter polychromator equipped with high-speed APD-based detection system can be easily stacked in a standard 19” rack (up to 15 per rack). Owing to compactness and ultra-low energy consumption of modern computers and analog-to-digital converters (ADC), both optical unit and data processing electronics with Gigabit optical Ethernet were combined in one stand-alone device (see Fig. 3). Such all-in-one design with fiber input and digital optical output is expected to be much more resistant to electromagnetic disturbances. Besides, the equipment can be integrated into the control/data acquisition system more easily. APDs are equipped with pHEMT-based (pHEMT – pseudomorphic High Electron Mobil-
Please cite this article in press as: E. Mukhin, et al., Hardware solutions for ITER divertor Thomson scattering, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.06.014
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3
4. Piezo actuators for ITER in-vessel
Fig. 3. Digital filter polychromator specially developed for TS.
Fig. 4. RRS signals measured in the 1 st spectral channel for different Nitrogen pressures.
ity Transistor) transimpedance preamplifiers specially designed for wideband, ultra-low noise amplification [9]. These preamplifiers are able to compensate slowly variable signals (background radiation); therefore, a full ADC dynamic range can be used to digitize TS signals. The integration time 3–4 ns was optimized for ∼3 ns FWHM TS signals. The acquisition system based on DRS4 chip [10] provides 12-bit digitizing with ∼0.25 mV amplitude resolution and ∼5 GHz sampling rate. Such high sampling rates combined with oscilloscope mode enable flexible processing of digitized signals as opposed to charge-sensitive analog methods. For sensitivity calibration we used rotational Raman scattering (RRS) generated by 1064 nm laser in nitrogen gas (4–1330 Pa). Fig. 4 shows scattering signal intensity versus N2 pressure. Signals for each pressure value were measured 100 times as shown by grey points. Mean values (red crosses in Fig. 4) demonstrate good linear correlation throughout the pressure range. The dispersion of Raman scattering signals (see Fig. 4) gives information about overall detection accuracy for different signal values, which includes superposition of statistical signal noise, amplifier noise and other disturbing influences on the signal. The measured level of intrinsic noise corresponds to ∼16 photoelectrons at the detector input (before avalanche gain) and demonstrates a dynamic range of 103 under the maximum digitized signal of 16,000 photoelectrons. According to this experiment, for signals of 100 photoelectrons and above, the noise of the RRS signal is determined mainly by statistical noise and constant excess noise factor. The excess noise factor F, defined as a ratio of the signal-to-noise ratios at the input of the multiplication process to that of the measured signal, was determined as F ∼ 4.7 with accuracy F ∼ 0.15 of the entire detector unit based on 1.5 mm APD Hamamatsu S11519 under the avalanche gain M = 100 for = 1064 nm.
Future fusion devices will require machinery to operate under vacuum conditions, radiation fluxes (up to ∼5 × 1019 neutron/cm2 fluence over ITER’s lifetime), heating up to 250 ◦ C and strong magnetic field (several Tesla). Conventional electric motors are not eligible for the in-vessel ITER applications (e.g. front divertor diagnostic rack, see Fig. 1), particularly under strong magnetic fields. Therefore, new actuator designs are required. The actuators based on the piezo effect seem ideal for fusion applications. Interestingly, that regular benefits of the piezo devices (submicron resolution, small size and noiseless operation) are less critical, while their high radiation and temperature resistance, as well as absolute insensitivity to magnetic fields become important. Mechanical movements and torques in the piezo actuators occur due to the frictional contact force between stator and rotor or slider. Since dry friction between similar materials in vacuum can easily lead to cold welding in the friction pair, specially matched dissimilar materials were coupled in the ITER intendent actuators. Different temperature coefficients of ceramic and metal linear expansions are compensated by gaps or by thermally-aged springs made of Inconel 718. Two types of piezoelectric actuators were developed for ITER applications – inertial stick-slip (SS) and ultrasonic (US, or resonant standing wave motor). SS type is based on a cyclic change of static and sliding friction resulting from applying an asymmetric sawtooth voltage to the piezoelectric element, which is friction coupled with a massive rotor. Because of inertia, the rotor must be put into motion during the saw-tooth slow slope and remains at its position on the fast slope. In the US motor, the rotor is driven in rotation with a piezoelectric vibrator that oscillates at a resonance frequency of 100–200 kHz and leads to an elliptical motion of a stator driving foot converted into the linear motion of a mover (or rotation of a rotor) pressed on the driving foot. The US piezo engine specificity is operation with fast transient responses and moderate loads as opposed to SS engines, which in some versions can move hundreds of kilograms but per minutes (or even per hours). In the developed piezo actuators, the following technical solutions have been implemented to satisfy all the ITER requirements: – High temperature piezo electric ceramic with the Curie temperature ≥300 ◦ C; – Alumina (Al2 O3 ), silicon nitride (Si3 N4 ) and ceramic mineral glue as insulating components; – Si3 N4 bearings with separators of AISI 304; – No lubricants and no soldering; – Spring elements of Inconel 718; – Friction pair in inertial version is Si3 N4 (rotor) and Inconel 718 (stator); – Friction pair in supersonic version is Si3 N4 (rotor) and piezo element (stator). Several designs of ultrasonic and inertial actuators were developed and successfully tested in vacuum at temperatures of 20 ◦ C and 200 ◦ C after annealing during 24 h at 350 ◦ C. Figs. 5a, b show the last versions of ultrasonic and inertial actuators. Fig. 6 demonstrates the measured relationship between torque and angular velocity at the start of acceleration of the US actuator testbed with the attached dummy shutter (the load torque of 0.01 N × m). The load has been chosen equal to that of the divertor TS FLM shutter. The maximal measured angular velocity is ∼8 rotations per second at 20 ◦ C vs ∼6.5 at 200 ◦ C. The maximal torque of ∼0.2 N × m and angular velocity 2 rotations per minute were measured for the SS actuator testbed featured with 6 piezo elements and the length of friction surface equal to circumference of Ø 40 mm rotor. The torque is expected
Please cite this article in press as: E. Mukhin, et al., Hardware solutions for ITER divertor Thomson scattering, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.06.014
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Fig. 5. Piezo actuators developed for ITER application based on Si3 N4 bearings of Ø 40 mm for SS actuator and Ø17 for US actuator. (a) US dimensions Ø54 × 15 mm; weight 160 g. (b) SS dimensions Ø84 × 24 mm; weight 750 g.
lated plasma background and to separate TS signals from signals of stray light scattered on nearby constructions. It is expected that the diode-pumped laser under development will improve the nearfield energy distribution in the laser footprint and further increase time between maintenance. TS polychromators based on bandpass-filters, high-speed APD-based detection system and ∼5-GHz, 12-bit ultra-low power ADC has an integrated design combining the optical unit, digitizer and compact computer unit for data processing with Gigabit optical Ethernet in one stand-alone device. Such a device with fiber input and digital optical output is expected to be quite resistant to electromagnetic disturbances. The experiments with Raman scattering show that for signals of 100 photoelectrons and above, the noise of the RRS signal is determined mainly by statistical noise and constant excess noise factor. The ultrasonic and inertial piezo motor designs satisfy all the ITER requirements. The piezo motor prototypes were tested for key specifications, although certain efforts are necessary for their specific adaptation and validation according to particular requirements. Acknowledgments This report supported in part by Rosatom (contract – N.4k.52.9Б.14.1002) was prepared as an account of work for the ITER Organization. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.
Fig. 6. Parameters of the developed US and SS piezo actuators: a – Acceleration of the US piezo actuator with the attached dummy shutter (load torque of 0.02 N × m); b – Relationship between torque and angular velocity for the developed SS actuator.
to be proportional and angular velocity – inversely proportional to rotor diameters of SS piezo actuators. It is obvious that, for both SS and US piezo actuators, rotors with larger diameter (circumference) are able to allocate more piezo elements, with respective increase of torques. Thus, to improve performance of piezo actuators (both SS and US), diameters and/or number of rotors should be adjusted. 5. Conclusions This paper focuses on innovative hardware – 1064 nm Nd:YAG; 946 nm Nd:YAG and 1047 nm Nd:YLF lasers; digital polychromator; US and SS piezo actuators. All these prototypes being developed for ITER divertor TS can readily be used for other ITER applications as well as in other tokamaks. The developed 1064 nm Nd:YAG laser operates at 100 Hz in steady state with pulse duration as low as ∼3 ns (FWHM). Such short lasing times help to reduce the accumu-
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Please cite this article in press as: E. Mukhin, et al., Hardware solutions for ITER divertor Thomson scattering, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.06.014